brubaker



Feb. 21, 1956 w. M. BRUBAKER 2,735,942

MASS SPECTROMETER Filed June 1, 1954 FIG/ 1 /7 l8 l9 SAMPLE g INLETAMPLIFIER 8 SENS/N6 NETWORK 1 i T0 3 VACUUM SYSTEM R F. A L E TOR FlaUNIFORM MA a: we

30 new g NON UN/FORM 32 3 MA c/vsr/c r/ao MAGNET POLE 4/ BOUNDARY. 45

ELECTRODE BOUNDARY ELECTRODE IN VEN TOR. WILSON M. BRUBAKER 44 MAGNETPOLE 40 ATTORNEY United States Patent MASS SPECTROMETER Wilson M.Brubaker, Arcadia, Califi, assignor to Consolidated EngineeringCorporation, Pasadena, Calif., a corporation of California ApplicationJune 1, 1954, Serial No. 433,752

7 Claims. (Cl. 250-413) This invention relates to mass spectrometry andparticularly to improvements in that form of mass spectrometry in whichmass separation is accomplished as a function of differences in theperiodicity of motion of ions of diifering mass to charge ratio.

The principle of mass spectrometry is, in general, one of separatingions produced from a sample to be analyzed as a function of the mass tocharge ratio of the ions and selectively collecting the separated ions.Ion separation may be accomplished in many ways, usually involvingapplication of magnetic or electrical fields, or both, to induce andtake advantage of characteristic differences in the movement of ions ofdilfering mass to charge ratio under the influence of such fields. Acollector electrode may be disposed in space so that under any given setof conditions only ions of a given mass to charge ratio will focus onand discharge at the collector electrode.

it has been found that ions subject to a uniform magnetic field and analternating electrical field normal to the magnetic field will move inspiral orbits about an axis parallel to the magnetic field. It is alsoknown that in such crossed magnetic and A. C. electrical fields, ions ofdiffering mass to charge ratio will exhibit different and characteristicperiods of movement about the axis as a function of the magnetic fieldstrength and the frequency of the alternating electrical field. Ions ofa given mass to charge ratio having a periodicity of motioncorresponding to the frequency of the alternating field will travelabout the axis in orbits of ever increasing radius. Ions of this givenmass to charge ratio are referred to as resonant ions.

All ions of mass to charge ratio different from the resonant mass willtravel about the axis in spiral orbits, the radii of which increase to amaximum, collapse back to the axis, etc. in successive cycles. Theseions are referred to as non-resonant ions. Non-resonant ions ofdilferent mass to charge ratio will travel in different orbits and willattain different maximum radii with those ions most closely approachingthe resonant mass attaining the greatest radial displacement from theaxis. By locating a collector electrode at a distance from the axisexceeding the maximum orbital radius of the non-resonant ions, theresonant ions can be selectively collected and measured. The maximumradius obtained by an ofl-resonant ion is proportional to the magnitudeof the applied R. F. voltage and inversely proportioned to the extent towhich the ion is off resonance.

By varying the frequency of the alternating field or the strength of themagnetic field, ions of differing mass to charge ratio will come intoresonance with the alternating field and all or a portion of the massspectrum may be scanned.

To avoid anomalous ion paths within the field, ions, are preferablyformed along an axis of the chamber parallel to the magnetic field andpreferably in the central region of the electrical field. This isconveniently accomplished by projecting an electron beam through thecrossed fields 2,735,942 Patented Feb. 21, 1%56 parallel to the magneticfield. The electron beam then coincides with the axis of rotation of theions.

The conventional practice in the operation of a mass spectrometer of thetype described is to develop a magnetic field of maximum uniformity. Ihave now found that many improvements in the operations of the massspectrometer result from a deliberate distortion of the magnetic fieldso that it decreases in intensity at increasing radii from the axis ofion formation.

The invention contemplates in a mass spectrometer the combinationcomprising an analyzer chamber, means for ionizing a gas admitted to thechamber, means for establishing an alternating electrical field acrossthe chamber, means for producing a non-uniform magnetic field across thechamber transverse to the electrical field, the strength of the magneticfield being at a maximum at a central region of the analyzer chamber anddecreasing outwardly thereof along any radius from the axis of ionformation, and a collector electrode disposed adjacent the boundary ofthe electrical field.

The improved operation of a mass spectrometer in accordance with theinvention will become apparent from the following detailed descriptiontaken in conjunction with the accompanying drawing in which:

Fig. 1 is a schematic sectional elevation of a mass spec trometer inaccordance with the invention, showing one form of magnet means fordeveloping the desired nonuniform magnetic field;

Fig. 2 is a schematic perspective of another embodiment of the inventionshowing a different form of magnet means;

Fig. 3 is a diagram illustrating the effect of a nonuniform magneticfield on ions immersed therein; and

Fig. 4 is a graphical portrayal of another effect of the non-uniformmagnetic field on the operation of the mass spectrometer.

The mass spectrometer shown in Fig. 1 comprises an envelope 10 having anexhaust outlet 11 for connection to a vacuum system (not shown) and asample inlet 12 for admitting a sample of gas to be analyzed into theenvelope. A plurality of electrodes 13, 14, 15, 16, 17, 18, 19 aredisposed in the envelope in spaced parallel relation. The outerelectrodes 13 and 19 may be solid plates and the inner electrodes 14,15, 16, 17 and 18 of ring type to provide a space for ion motion withinthe region defined by the electrodes. The several field-formingelectrodes are connected across a voltage divider 20, which is in turnconnected to an R. F. oscillator 21 by means of which an alternatingvoltage is impressed on the several electrodes so as to establish withinthe envelope a substantially uniform alternating electrical field whichis of symmetrical configuration about the center electrode 16.

A conventional electron gun 22 and an electron target 23 are disposedadjacent opposite sides of the envelope to direct an electron beamacross the envelope transversely of the electrical field andsubstantially at the midpoint thereof. The beam may be directed throughcollimating apertures in the median electrode 16.

The entire envelope is immersed in a magnetic field established betweenmagnet pole pieces 24, 24A. These magnet poles are of uniqueconfiguration for this type of mass spectrometry in the provision ofconical pole faces with the apex of the pole faces being in line Withthe axis of symmetry of the envelope and the electronbeam. As aconsequence of this configuration of the pole pieces the magnetic fieldacross the envelope normal to the direction of the electrical field isat a maximum at the axis of symmetry of the system and decreases withincreasing radii outwardly of the axis of symmetry. A collectorelectrode 25 is mounted adjacent a boundary of the electrical field andis connected to a conventional amplifying and sensing network 26.

, ions.

.in Fig. 3.

The operation of the mass spectrometer shown in Fig. l is, in general,similar to that of other conventional mass spectrometers of this type. Agas sample introduced to the envelope through inlet 12 is ionized by theelectron beam established between the gun 22 and the target 23. Underthe influence of the transverse magnetic and electrical fields, the ionsmove in the fields in essentially spiral paths about an axis which, inthis instance, coincides with the electron beam and thus the origin ofthe As mentioned above, ions of a given mass may be made spiraloutwardly from the axis of rotation until they are collected at thecollector electrode 25. Ions of other than the given mass follow spiralpaths, the radii of which increase to a maximum for each such othervmass and then collapse back to the origin in successive cycles. Byvarying the frequency of the alternating field different ion masses maybe caused to reach the collector electrode.

Fig. 2 shows schematically another form of mass spectrometer in whichenvelope is immersed in a magnetic field established by pole pieces 31,32. The envelope 33, although not apparent from the drawing, is providedwith field-forming electrodes, collector electrode, evacuating outlet,and sample inlet identical to the system shown in Fig. l. The onlydifference in the mass spectrometer of Fig. 2 is that the magnet poles31 and 32 are of peaked configuration, so that the magnetic fieldestablished there by decreased with increasing radius along only oneaxis of the envelope, this axis being parallel to the direction of theelectrical field. The operation of the mass spectrometer of Fig. 2 isthe same as that of Fig. 1 and as described above.

Satisfactory resolution can be accomplished in this conventional type ofmass spectrometer having a uniform magnetic field only by causing theions to make a large number of turns about the point of ion formation.As a consequence some means must be provided for accelerating the ionstoward the median plane. The median plane is defined for this purpose asa plane lying midway between the magnetic pole pieces and parallel tothe direction of the electric field. Without some such means foraccelerating the ions toward the median plane, their initial velocity ina direction parallel to the magnetic field will tend to carry themoutside the influence of the electric field in the time required to makethe large number of turns necessary for good mass resolution. The termtrapping is applied to the phenomenon of accelerating ions which are noton the median plane toward the median plane.

When a mass spectrometer of this type is operated in a uniform magneticfield as is presently the conventional practice,-trapping must beinduced by electrostatic action since the lens action of the alternatingfield has been found to be inadequate for this purpose. Although highlyeffective trapping action can be obtained by means of an electrostaticfield, it has been found in practice that such a field is difficult tocontrol at optimum values which are dependent and complicated inunpredictable ways upon the parameters of the frequency and magnitude ofthe alternating field, the mass of the ions, pressure within theenvelope, anode current, etc.

I have now found that trapping can be accomplished by means of thenon-uniform magnetic field established by shaping the magnet poles, asfor example in the manner illustrated in Figs. 1 and 2. The manner inwhich such a non-uniform magnetic field accomplishes the desiredtrapping action is illustrated schematically In the figure, magnet polesand 41 and boundary electrodes 42 and 43 define a region of crossedmagnetic and electrical fields of the type developed in the instrumentillustrated in Fig. l. The lines of force of the magnetic fielddeveloped by magnet poles of this type are normal to the pole faces andhence assume the configuration shown in the drawing at 44. If an ion 45is moving perpendicularly to the plane of the drawing,

the instantaneous force exerted on the ion by the magnetic field at theposition shown is represented by the arrow X. This force resolves intovectors r and y, r being the normal radial force to which the ion wouldbe subjected in a uniform magnetic field and q being the trapping forcewhich is unique to the non-uniform magnetic field established by theconical or peaked pole pieces 40 and 41.

From the foregoing description it is apparent that the non-uniformmagnetic field results in a simplification of the instrument byeliminating the requirement of an electrostatic trapping field and alsoimproves the sensitivity of the instrument by preventing motion of ionsalong the axis of the magnetic field. A further advantage of thenon-uniform field is realized in improved resolution.

When the magnetic field varies with radius there is only one radius atwhich a given ion is strictly resonant, that is, at which the naturalfrequency of rotation in the magnetic field is equal to the frequency ofthe applied alternating field. At this given radius the phase anglebetween the position vector of the ion relative to the electric vectoris constant. At smaller radii the ion rotates faster than the electricvector and at larger radii it rotates slower. The pulsating electricalfield is really the combination of two rotating electrical vectors eachof half the peak voltage intensity and each rotating in oppositedirections. A positive ion near resonance rotates with one of thesevectors insensitive to the other while a negative ion is oppositelyresponsive. For this reason the energy gained by an ion each revolutioncan be expressed as follows:

e=E0qp1r sin (p where e is the energy;

q is the charge on the ion;

E0 is the maximum field magnitude;

p is the radius of the nearly circular path of the ion;

and

(p is the phase angle between p and E0 From this equation it is apparentthat the ion gains energy so long as 1r 0. If the resonant radius for agiven ion is less than the collector radius, the ion will at firstrotate faster than the electric vector. Provided the ion reaches theresonant radius before =1r it will continue to gain energy and radiusand so will enter a region where will decrease. When p=7r/2 the ion willbe in the proper phase relation to gain energy at a maximum rate forthat radius. However, it now will be in a region Where it rotates slowerthan the electric vector and unless it reaches the collector before pbecomes 0 it will start losing energy and return toward the origin.

As a consequence of the foregoing relationships, when the magnetic fieldis non-uniform is accordance with the invention there is a threshold ofR. F. voltage below which the ion will not reach the collector, evenwhen the applied voltage is of the most favorable frequency. Forvoltages just above this threshold, the band of frequency through whichthe ion can be accelerated to reach the target is quite narrow. Thus,the non-uniform magnetic field results in narrow peaks as the frequencyis scanned, while at the same time the level of radio frequency voltageis high and the total number of turns is moderate. This is desirablesince the smaller the number of turns required for resonant ions toreach the collector the smaller the adverse effect of space and surfacecharges. With the uniform magnetic field, on the other hand, narrowpeaks are obtainable only by reducing the R. F. voltage to a relativelylow value and correlatively increasing the number of turns required bythe resonant ion to reach the target. Under this circumstance anunfavorable relationship between spurious gradients due to space orsurface charges and the gradients due to the applied R. F. voltages isencountered.

Another important property of the non-uniform magnetic field isillustrated graphically in Fig. 4 in which the band pass of theinstrument is plotted against the magnitude of the R. F. voltage. In thefigure, curve A shows the relationship of band pass to R. F. voltagewith a uni form magnetic field, and curve B shows this relationship in anon-uniform magnetic field. From this figure it is apparent that for agiven R. F. voltage operation with a non-uniform magnetic field of theconfiguration disclosed will result in a relatively narrower band passcharacteristic. The band pass characteristic of the type of massspectrometer is proportional to peak width obtained as frequency isscanned.

The use of a non-uniform magnetic field in a mass spectrometer of thetype involving crossed magnetic and alternating electrical fields thusimproves the operation of this type of instrument with respect toimproved sensitivity, the formation of narrow and well defined ion peaksas a consequence of characteristic of threshold voltage, with resultinghigh resolution. Moreover, the instrument is simplified by theelimination of complex control circuitry normally required for anelectrostatic trapping field at the same time the foregoing advantagesaccrue.

Two forms of magnetic means have been shown for developing a desirednon-uniform field. It is important in accordance with the invention thatthe magnetic field decrease from a maximum at the axis of ionization toa minimum at the boundary of the electrical field. With conically shapedmagnet means, as shown in Fig. 1, the maximum magnetic field isdeveloped along a line coinciding with the ionizing electron beam andthe magnetic field decreases along any radius from this line. With thepeaked magnetic poles shown in Fig. 2, the maximum magnetic field isestablished in a plane intersecting the electron beam and lyingtransverse to the electrical field and the magnetic field decreasesalong radii extending from this plane toward the boundary of theelectrical field. Other magnet shapes may be employed to accomplishthese results. As for example the conical surfaces of the magnet meansshown in Fig. 1 and the sloping surfaces of the magnet shown in Fig. 2may be either convex or concave. It is important only that the magneticfield decrease symmetrically about the point of maximum magnetic field.

I claim:

1. In a mass spectrometer the combination comprising an analyzerchamber, means for ionizing a gas admitted to the chamber within arestricted region of the chamber, means for producing a high frequencyelectrical field across the chamber, means for producing a non-uniformmagnetic field across the chamber transverse to the electrical field,the strength of the magnetic field being at a maximum in a centralregion of the analyzer chamber and decreasing outwardly thereof in thedirection of the electrical field, and a collector electrode disposed inthe chamber and spaced from the region of ion formation.

2. In a mass spectrometer the combination comprising an analyzerchamber, means for ionizing a gas admitted to the chamber within arestricted region of the chamber, means for producing a high frequencyelectrical field across the chamber, a pair of magnet poles disposed onopposite sides of the chamber and having substantially conical polefaces for producing a non-uniform magnetic field across the chambertransverse to the electrical field, and a collector electrode disposedin the chamber and spaced from the region of ion formation.

3. In a mass spectrometer the combination comprising an analyzerchamber, means for ionizing a gas admitted to the chamber within arestricted region of the chamber,

means for producing a high frequency electrical field across thechamber, a pair of magnet poles disposed on opposite sides of thechamber and having pole faces which taper outwardly from a centralportion for producing a non-uniform magnetic field across the chambertransverse to the electrical field, and a collector electrode disposedin the chamber and spaced from the region of ion formation.

4. In a mass spectrometer the combination comprising an analyzerchamber, means for directing the ionizing electron beam across thechamber, means for producing an R..F. electrical field across thechamber transverse to the electron beam, means for producing anon-uniform magnetic field across the chamber parallel to the electronbeam in the region of the beam, the strength of the magnetic field beingat a maximum along the electron beam and decreasing outwardly thereofalong radii extending in the direction of the electrical field, and acollector electrode disposed in the chamber and spaced from the electronbeam.

5. In a mass spectrometer the combination comprising an analyzerchamber, means for directing an ionizing electron beam across thechamber, means for producing an R. F. electrical field across thechamber transverse to the electron beam, means for producing anon-uniform magnetic field across the chamber parallel to the electronbeam in the region of the beam, the strength of the magnetic field beingat a maximum along the electron beam and uniformly decreasing outwardlythereof along radii extending in the direction of the electrical field,and a collector electrode disposed in the chamber and spaced from theelectron beam.

6. In a mass spectrometer the combination comprising an analyzerchamber, means for directing an ionizing electron beam across thechamber, means for producing an R. F. electrical field across thechamber transverse to the electron beam, means for producing anon-uniform magnetic field across the chamber parallel to the electronbeam in the region of the beam, the strength of the magnetic field beingat a maximum at the electron beam and decreasing along any radiioutwardly of the beam, and a collector electrode disposed in the chamberand spaced from the electron beam.

. 7. In a mass spectrometer the combination comprising an analyzerchamber, means for directing an ionizing electron beam across thechamber, means for producing an R. F. electrical field across thechamber transverse to the electron beam, means for producing anon-uniform mag-.

netic field across the chamber parallel to the electron beam in theregion of the beam, the strength of the magnetic field being at amaximum in a plane intersecting the electron beam transverse to theelectrical field and decreasing outwardly of this plane in oppositedirections, and a collector electrode disposed in the chamber and spacedfrom the electron beam.

No references cited.

1. IN A MASS SPECTROMETER THE COMBINATION COMPRISING AN ANALYZERCHAMBER, MEANS FOR IONIZING A GAS ADMITTED TO THE CHAMBER WITHIN ARESTRICTED REGION OF THE CHAMBER, MEANS FOR PRODUCING A HIGH FREQUENCYELECTRICAL FIELD ACROSS THE CHAMBER, MEANS FOR PRODUCING A NON-UNIFORMMAGNETIC FIELD ACROSS THE CHAMBER TRANSVERSE TO THE ELECTRICAL FIELD,THE STRENGTH OF THE MAGNETIC FIELD BEING AT A MAXIMUM IN A CENTRALREGION OF THE ANALYZER CHAMBER AND DECREASING OUTWARDLY THEREOF IN THEDIRECTION OF THE ELECTRICAL FIELD, AND A COLLECTOR ELECTRODE DISPOSED INTHE CHAMBER AND SPACED FROM THE REGION OF ION FORMATION.